414 Encyclopedia of the Solar System
with depth in Saturn’s interior. The inferred density struc-
ture is consistent with this widely accepted explanation for
Saturn’s low atmospheric helium abundance. If helium is
presumed to be uniformly depleted from the outer molec-
ular envelope of the planet, it can be self-consistently ac-
counted for in the deeper interior. The unmixed helium may
have actually been removed from molecularandmetallic re-
gions of the planet and settled down on top of the core. The
models lack the sensitivity to confirm that this is definitely
the case, however.
The inferred interior structure of Saturn is most con-
sistent with the giant planet formation scenario known as
nucleated collapse. In this scenario, a nucleus of rock and
ice first forms in the solar nebula. When the nucleus has
grown to about 10 M⊕, the gas of the nebula collapses down
upon the core, thus forming a massive hydrogen–helium
envelope surrounding a rock/ice core. Planetesimals that
accrete later in time cannot pass through the thick atmo-
sphere surrounding the core. Instead, they break up and
dissolve into the hydrogen–helium envelope. This scenario
accounts for both the core of the planet and the enrichment
of heavy elements in the envelope. It is possible that Jupiter
formed via a different mechanism, such as the direct gravi-
tational collapse of nebular gas. It is perhaps more likely that
both Jupiter and Saturn had larger cores that were partially
dredged up by convective plumes over the past 4.5 billion
years. This mechanism could plausibly be more efficient
in the hotter interior of Jupiter, where convection is more
vigorous.
5.4 Uranus and Neptune
Before theVoyagerencounters, Uranus and Neptune were
assumed to have similar interior structures. This assump-
tion was well justified given their similar radii, masses, at-
mospheric compositions, and location in the outer solar sys-
tem. Uranus and Neptune were modeled as having three
distinct layers: an inner rocky core, a large icy mantle, and a
methane-rich hydrogen–helium atmosphere. Little more
could be said with precision because their atmospheric
oblateness and interior rotation rates were not accurately
known.
Upon its arrival at Uranus in 1986 and Neptune in 1989,
Voyager 2provided the measurements needed to constrain
interior models and provide individual identities for each
planet.Voyagerobserved the structure of the magnetic field
of both planets and measured their rotation rates. In both
cases, the fields were off-center, tilted dipoles of similar
strengths.Voyageralso measured the higher order compo-
nents of the gravitational fields of both planets. The abun-
dance of carbon in both atmospheres is about 30 times the
solar value. Although Uranus and Neptune have similar
radii and masses, the differences are such that the mean
density of Neptune is 24% higher than the mean density of
Uranus.
Voyagerdata revealed that, though similar, the interior
structures of the two planets are not identical. As with
Jupiter and Saturn, 2 provides information on the distri-
bution of mass inside each planet. If Uranus and Neptune
had a similar distribution of mass in their interiors, their 2
parameters would be similar. As Table 1 shows, for Uranus
2 =0.119, whereas for Neptune 2 =0.136. Neptune’s
larger value of 2 implies that it is less centrally condensed
than Uranus. Models show that this difference can be un-
derstood in terms of equal relative amounts of ice, rock,
and gas that are simply distributed differently within the
two planets. The two planets also follow virtually the same
pressure–density law, another indication that they have very
similar composition and structure.
Models (Fig. 9) of Uranus and Neptune’s interior be-
gin with a hydrogen-rich atmosphere that extends from the
observable cloud tops to about 85% of Neptune’s radius
and 80% of Uranus’. The composition in this region does
not vary significantly from the hydrogen-rich atmospheric
composition. Near 0.3 Mbar and 3000 K (0.85RNeptuneand
0.80RUranus), the density rises rapidly to over 1000 kg m−^3.
The density then increases steadily into the deep interior
of both planets, where the pressure reaches 6 Mbar at
7000 K. The variation of density with pressure in this re-
gion is very similar to that found in the laboratory shock
wave experiments on the artificial “icy” mixture known as
synthetic Uranus. The composition of this region is thus
undoubtedly predominantly icy. However, since the den-
sity of rock/ice/gas mixtures can mimic the density of pure
ice, the exact composition cannot be known with precision.
Any hydrogen present in the deep interior would be in the
metallic phase.
Interestingly, Uranus and Neptune models that do not
have rock cores can be constructed. Other models with
cores as large as 1M⊕are also consistent with the available
data.
The total mass of hydrogen and helium in Uranus and
Neptune is about 2M⊕, compared to about 300M⊕at
Jupiter. Given the relatively small amounts of gas com-
pared to ices in Uranus and Neptune, these planets are
aptly termed ice giants, whereas Jupiter and Saturn are in-
deed gas giants.
Shock compression measurements show that the fluids
of the hot, ice-rich region of Uranus and Neptune are ex-
pected to be substantially ionized and dissociated. The large
electrical conductivities of such fluids, coupled with the
modest convective velocities predicted for the interiors of
Uranus and Neptune, can generate and sustain the observed
magnetic fields of the planets. One possible explanation for
the complexity of their magnetic fields is that the electrically
conductive region of these planets is comparatively close
(within about 4000 km) to the cloud tops, a consequence of
the ionization behavior of water, ammonia, and methane.
This is consistent with the trend in field complexity seen at
Jupiter and Saturn.